The invention relates to a method of poling a sample of ferroelectric material, comprising a plurality of ferroelectric layers arranged in a stack, so as to induce bulk piezoelectricity. In particular, but not exclusively, the invention relates to a method of poling multilayer ferroelectric samples of the type suitable for use in piezoelectric actuators for fuel injection systems for internal combustion engines.
FIG. 1 is a schematic view of a piezoelectric actuator 2 of the type commonly used to actuate a valve needle of a fuel injector 5 (shown in FIG. 2) for a compression ignition internal combustion engine. The actuator 2 includes a poled piezoelectric stack 2a having a plurality of piezoelectric layers 4 separated by a plurality of internal electrodes forming positive and negative electrode groups 6a and 6b respectively. The figure is illustrative only and in practice the stack would include a greater number of layers and electrodes than those shown. Arrows, exemplified by arrows 4a and 4b, between adjacent interdigitated electrodes 6a, 6b indicate the dominant direction of remanent polarisation of the dipoles contained in the piezoelectric layers 4; wherein, the arrow-head of each arrow 4a, 4b indicates the position of the negative pole of each dipole, and the arrow-tail indicates the position of the positive pole of each dipole. The arrows are illustrative only and in practice there would be many more dipoles than indicated in the figures.
The electrodes of the positive group 6a are interdigitated with the electrodes of the negative group 6b, with the electrodes of the positive group 6a connecting with a positive external electrode 8a and the electrodes of the negative group 6b connecting with a negative external electrode 8b. The positive and negative external electrodes 8a, 8b receive an applied voltage, in use, that produces an intermittent electric field between adjacent interdigitated electrodes 6a, 6b. The intermittent electric field rapidly varies with respect to its strength. In turn, this causes the stack 2a to extend and contract along the direction of the applied field.
A lower end cap 10b is adjacent to the lowermost piezoelectric layer 4 of the stack 2a and an upper end cap 10a is adjacent to the uppermost piezoelectric layer 4 of the stack 2a. The lower end cap 10b is coupled to an injector valve needle 7 (shown in FIG. 2), either directly or through an intermediate mechanical and/or hydraulic coupling. Thus, as the stack 2a extends and contracts upon application of the electric field, the injector valve needle 7 is caused to move to control injection of pressurised fuel into an associated engine cylinder (not shown).
Referring to FIG. 2, in order to prevent injection of fuel into the cylinder, the injector valve needle 7 securely abuts an injector nozzle seating 15b; thereby preventing fuel from passing through fuel channels 15a in the nozzle 15. This is achieved by applying a voltage of 200V to the electrodes of the positive group 6a which causes the stack 2a to extend. The electrodes of the negative group 6b are maintained at 0V. Due to fuel injection taking a relatively short period of time, the fuel injector needle 7 is engaged with the associated seating 15b in the aforementioned manner for approximately 95% of the fuel injector's operating cycle.
To inject fuel into the cylinder the voltage applied to the electrodes of the positive group 6a is rapidly reduced, thus causing the stack 2a to contract. The amount that the voltage is reduced is dependent on the pressure of the fuel. For example, at a minimum pressure of around 200 bar (such as when the engine is idling) the voltage applied to the electrodes of the positive group 6a will drop to 20V, and at a maximum pressure of around 2000 bar the voltage applied to the electrodes of the positive group 6a will drop to −20V, briefly making the electrodes of the positive group 6a negative.
In order for the piezoelectric actuator 2 to behave in the aforementioned manner, it is necessary to pole the piezoelectric stack 2a. One known process of poling the piezoelectric stack 2a will be described with reference to FIGS. 3 to 5.
An example of an unpoled multilayer ferroelectric sample of which the piezoelectric stack 2a is comprised is shown schematically in FIG. 3. A multilayer structure 3 is formed from a plurality of relatively thin ferroelectric ceramic layers 4, as in FIG. 1. An example of a ferroelectric material is lead zirconate titanate, also known by those skilled in the art as PZT.
The multilayer structure 3 is poled by applying a potential difference across the positive and negative external electrodes 8a, 8b which, in turn, apply the potential difference across the internal groups of positive and negative electrodes 6a, 6b. In order to achieve poling of the dipoles contained within the piezoelectric material, the dipoles must be exposed to an electric field large enough to cause permanent crystallographic realignment and dipole reorientation. The minimum electric field strength necessary to affect this change is referred to as the “coercive” field strength. Due to the alternating polarity of the internal electric field, the poling direction of the dipoles within the piezoelectric material alternates throughout the structure, as indicated by the arrows, exemplified by arrows 4c and 4d, orthogonal to the internal groups of positive and negative electrodes 6a, 6b in FIG. 4.
A ferroelectric multilayer will only be poled where it is exposed to the coercive electric field. As shown in FIG. 5, once the coercive field has been applied to the multilayer structure 3, the central regions of the ferroelectric layers 4 contained between adjacent oppositely charged electrodes 6a, 6b (shown as central region 14) are poled. However, the ends of the piezoelectric layers 4 contained in the side regions 12 are not subjected to an electric field because adjacent electrodes in these regions are at the same potential; thus, the piezoelectric material in the side regions 12 remains unpoled. This gives rise to a ferroelectric strain discontinuity between the regions of poled and unpoled ceramic, placing the unpoled material in tension and the poled material in compression. This is because when a ferroelectric material is poled, the crystallographic realignment creates a permanent and temporary elongation along the axis of the applied field. In the multilayer structure 3 this elongation only occurs within the interdigitated central region 14. As a result, the poled material is clamped which may reduce actuator stroke. The unpoled material tends to fracture and crack due to the tensile forces created within it by the elongation of the central region 14. These cracks 16 are identified in FIG. 1. Furthermore, longitudinal distortion may occur if the clamping of the poled material by the unpoled material in the side regions 12 is not uniform.
When the actuator is in use, the cracks 16 are worsened due to the repeated tensile strain imposed by the rapidly intermittent electric field which further fatigues the composite structure in the margins. In addition to the temporary elongation of the stack 2a produced by the intermittent field, the permanent and temporary straining of the poled central region 14 causes the end caps 10a, 10b to experience lateral compression and bending which manifests itself as doming of the end caps 10a, 10b; illustrated by the shaded areas of the end caps 10a, 10b. 
Ferroelectric materials and their associated poling are discussed further in the Applicant's co-pending patent application EP 1 516 373 A2.
It is an object of the present invention to provide a method of poling a sample of ferroelectric material which removes or alleviates the aforementioned problems.